Gel electrophoresis is commonly used to separate biomolecules such as deoxyribonucleic acids (DNA), ribonucleic acids (RNA) and proteins by molecular size. Generally, to perform electrophoresis a separation medium such as a polymeric gel is formed in a glass tube or between spaced glass or plastic sheets such that the gel forms a porous matrix with pores of a desired size depending upon the choice of gel and biomolecules to be separated. The tube or plates are placed in a container along with anode and cathode elements positioned at the top and bottom of the gel. Samples are placed in receptacles formed in the top of the gel which, during electrophoresis, result in several sample channels in the gel. Electrophoretic buffer solutions containing conductive ions are added to the container for electrical contact between the gel, samples and the cathode and anode. A voltage drop is then applied across gel causing the samples to migrate through the gel separating into bands depending upon the size of the biomolecule in relation to the porous structure of the gel. The gel can then be removed from the system and manipulated and analyzed using methods such as, e.g. blotting or gel staining.
Commercial gel electrophoresis systems are known in the art and include XCell II Mini-Cell (Novex), XCell Surelock Mini-Cell (Invitrogen), SE200 Series Min-gel System (Hoefer Pharmacia) and the Mini-PROTEAN II Electrophoresis Cell (Bio-Rad).
Because it is often desirable to analyze biomolecules separated during gel electrophoresis, many techniques exist in the art for the transfer of a biomolecule in a gel electrophoretic band (spot) to an analytical device such as mass spectrometer to identify or further analyze the biomolecule. One such method includes the step of transferring the biomolecule from an electrophoresis gel to a separate gel plug (electroelution) which can then be further manipulated for analysis by, for example, destaining and extraction. The prevalent approach to such identification includes the excision of the gel band followed by the in situ proteolysis of the spot and extraction of the biomolecules by either diffusion or electrophoresis. Attempts have been made in the art to perform the electroelution on the electropherogram directly, i.e. without slicing the gel. Attempts have also been made to include intermediate steps such as testing for homogeneity after electroelution, identification of the protein by its intact mass independently of proteolysis and identification of a protein on the basis of its peptide map. Other methods in the art include elution or electroelution of biomolecules into a collection chamber, direct positioning of gel pieces containing a band onto the receptacle of a mass spectrometer, and blotting of the gel membrane followed by direct mass spectrometric analysis of the blot.
The aforementioned techniques require extensive time and effort because the separated biomolecules cannot be sequentially electroeluted for subsequent manipulation or analysis. Because of the foregoing reasons there is a desire in the art for a method and apparatus for the sequential electroelution of multiple biomolecule bands contained in a separation gel.